Numerical Study On Secondary Flow Evolution Law And Boiling Heat Transfer Characteristics In The Spiral Tube

As an efficient and compact heat exchange element,spiral tube is widely used in important industrial equipment such as coiled-wound heat exchanger.Compared with straight pipe,spiral pipe can induce secondary flow perpendicular to the main flow direction.Secondary flow is the main reason for heat transfer enhancement in spiral tube,but it is greatly affected by geometric parameters and working conditions.The evolution process and heat transfer mechanism of secondary flow in spiral tube are complex,especially in boiling process.Therefore,this paper used FLUENT software to simulate and analyze the single-phase and steam-water two-phase boiling heat transfer process in spiral tube.The main research contents and conclusions are as follows:(1)The calculation models of single-phase and two-phase flow boiling heat transfer in spiral tube were established,and the relevant simulation methods were set up.The independence of the grid was verified,and the same model as that in the literature was established.The boiling heat transfer of single-phase and two-phase flow under the corresponding conditions were calculated.The reliability of the simulation model was verified by comparing with the experimental data in the literature.(2)In previous studies,the ratio of velocity to average velocity(V/Va)was often used to judge the flow characteristics in spiral tube.However,V/Va can not reflect the velocity change of velocity boundary layer fluid affected by spiral tube structure.Therefore,this paper defined a new dimensionless number V/Vs accelation intensity;that is,the velocity ratio of helical tube and inclined straight tube at the same position,and described the characteristics and evolution law of single-phase secondary flow in spiral tube.Results show that V/Vs can reflect the secondary flow intensity.In the development section of secondary flow,the secondary circumfluence/circulation flow is mainly located in the upper left side of the cross section.In the fully developed section,the secondary flow is distributed in the upper left and lower right regions,and the secondary flow intensity in the upper left region is greater than that in the lower right region.V/Vs and secondary flow intensity increase with the increase of Re,pitch P and diameter d,and decrease with the increase of winding diameter D.The longer the Re and diameter d are,the longer the secondary flow developing section is.The larger the winding diameter D is,the shorter the secondary flow developing section is.(3)The secondary flow and boiling heat transfer mechanism in the subcooled boiling section,nucleate boiling section,liquid film convective boiling section and evaporation drying heat transfer section of spiral tube were analyzed and discussed respectively.Results show that the characteristics of secondary flow in subcooled boiling section and evaporation drying section are the same as those of single-phase secondary flow.In nucleate boiling and film convective boiling,the secondary flow in the vapor region is affected by both curvature and buoyancy,while the liquid water is only affected by curvature.The secondary flow intensity in the steam region is much stronger than that in the liquid water region.There is only one maximum value of fluid temperature near the wall in subcooled boiling section.The circumferential temperature distribution of nucleate boiling section is "M" type,and that of liquid film convective boiling section is "W" type.The circumferential temperature distribution in the steam drying section is similar to that in the subcooled boiling section,but the temperature fluctuation is large due to the influence of wet steam.In the subcooled boiling section,the surface heat transfer coefficient increases with the increase of axial angle,and reaches the saturation temperature with the increase of mainstream temperature.In the nucleate boiling section,the surface heat transfer coefficient increases sharply,reaches the maximum and then decreases gradually.In the convective boiling section,the surface heat transfer coefficient decreases continuously.The surface heat transfer coefficient drops precipitously until the evaporation section is dry.(4)The effects of geometric parameters and boundary conditions on the two-phase secondary flow and boiling heat transfer characteristics in spiral tube were analyzed.The results show that the angle between vapor-liquid interface and horizontal line increases with the increase of Re and P,and decreases with the increase of d and D.The secondary flow intensity increases with the increase of Re and P,and decreases with the increase of d and D.The surface heat transfer coefficient h increases with the increase of Re and heat flux q,and decreases with the increase of d.The length of boiling section increases with the increase of Re and decreases with the increase of q.